A colorless solution contains one cation. When dilute hydrochloric acid is added, a white precipitate forms that dissolves upon heating. Which cation is most likely present?
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Correct answer
B. Pb2+
Principle or equation
Lead(II) ions form a white precipitate of PbCl2 with chloride ions; this precipitate is soluble in hot water. Silver chloride is also white but does not dissolve on heating. Barium chloride is soluble, and copper(II) solutions are blue.
Why this answer is correct
Add Cl- to the unknown solution. Pb2+ gives PbCl2(s), a white precipitate that dissolves when heated because its solubility increases with temperature. AgCl is also white but remains insoluble on heating. Ba2+ does not precipitate with Cl- under these conditions. Cu2+ would give a blue solution, not colorless.
Example
A solution suspected to contain Pb2+ is treated with dilute HCl. A white precipitate forms; heating the mixture causes the precipitate to disappear, confirming Pb2+.
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